1. Field of the Invention
The present invention relates to a method for forming a metal oxide film using a sputtering process. Specifically, the present invention relates to a method for forming a metal oxide film made of a bolometer material to be used for a two-dimensional infrared imaging apparatus of the non-cooling type which performs the detection of infrared radiation through the use of variations in resistance with temperature variations.
2. Description of the Related Art
Generally, metal oxides having comparatively large variations in resistance with temperature (i.e., temperature coefficient resistances: TCRS) have been provided as bolometer materials to be used in the non-cooling type two-dimensional infrared imaging apparatus. Among them, vanadium oxide (VOx) is known as such a material which is able to easily obtain a high TCR, and is thus formed as a thin film to be used as a sensor of the temperature-measuring apparatus or the non-cooling type two-dimensional infrared imaging apparatus.
The method for forming such a metal oxide thin film typically includes a physical deposition process, such as vacuum evaporation, or a sputtering process. Among them, the sputtering process is used extensively because of its excellent productivity. For instance, the method for forming a VOx thin film using a sputtering process is disclosed in Jerominek et al., Optical Engineering, a second paragraph of page 2093, vol. 32-9, 1993.
In order to form the above metal oxide film using the sputtering process, a reactive sputtering process may be employed. In the reactive sputtering process, a cathode target made of a pure metal or a metal oxide is arranged in a sputtering chamber and an anode stage is then arranged to face the cathode target, followed by placing a wafer on the anode stage. Subsequently, argon (Ar) is introduced as a sputtering gas and oxygen (O2) is introduced as a reaction gas in the sputtering chamber, followed by subjecting to an electric discharge process. Consequently, a metal oxide film can be deposited on the surface of the wafer. By the way, for the use of a metal oxide film in the two-dimensional infrared imaging apparatus or the like, it is very important to make the thin film on the wafer so that the characteristics of the thin film are extremely consistent across the surface of the wafer.
Regardless of such an importance, there is a problem that the characteristics of the metal oxide film on the surface of the wafer become uneven when the metal oxide film is deposited on the wafer using the conventional reactive sputtering process. In addition, there is another problem that the characteristics of the metal oxide film formed on the wafer may be varied depending on the type of the sputtering apparatus, i.e., a sheet-fed type sputtering apparatus in which wafers are processed one by one or a batch type sputtering apparatus in which plural wafers are simultaneously processed.
The reason of causing such a problem is the generation of positive self bias potential on the surface of the wafer. As the surface of the wafer is covered with an interlayer film with sufficient electrical insulation (e.g., a silicon oxide film (SiO2) or a silicon nitride film (SiNx)), the surface of the wafer may become positively charged as a result of dielectric polarization caused in the insulation film when the discharge is initiated after placing the wafer on the anode stage. Similarly, the surface of the wafer may become positively charged to have a self bias potential in spite of a smaller insulation value compared with that of the above insulation film in a case where a metal oxide film to be formed by the sputtering process has an insufficient electrical conductivity compared with that of a pure metal. Such a metal oxide film may be, for example, a semiconductor metal oxide film with a resistibility of 1xc3x9710xe2x88x925 to 100 xcexa9xc2x7m.
We will explain the self bias potential as follows. FIG. 1 is a schematic perspective diagram that shows the inside of the conventional sputtering apparatus where a wafer 1 is placed on an anode stage 4. In this figure, also, there is a graph that schematically shows the distribution of self bias potentials on the wafer, in which the locations on the anode stage are plotted on the horizontal axis and the self bias potentials corresponding to the respective locations are plotted on the vertical axis. By the way, the horizontal axis of the graph also corresponds to the positions of the anode stage 4 and the wafer 1. In this case, the anode stage 4 has a diameter of 13 inches (33 cm). As shown in FIG. 1, uneven self bias potentials are generated on the anode stage 4 because of the presence of an insulating deposition film 3 such as a metal oxide film being deposited on the wafer 1 mounted on a wafer-mounting area of the anode stage 4 and a redundant electrode area external to that wafer 1, and so on.
The self bias potential generated on the anode stage 4 functions as an opposite force to be exerted on argon ions which are ionized positive ions in plasma. Thus, the self bias potential decreases the speed of argon ions flying to the anode stage 4 and prevents them from their arrivals on the anode stage 4. Therefore, the influence (self-purification) of bombardment with argon ions to a metal oxide film which is depositing on the wafer 1 mounted on the anode stage 4 becomes out of balance when the self bias potentials are unevenly distributed on the wafer 1. Consequently, the characteristics of such a metal oxide film to be obtained (i.e., the composition and molecular structure of the film) becomes inconsistent over the entire surface of the wafer 1, so that uniformity in characteristics of the wafer surface (e.g., a resistance value thereof) becomes deteriorated.
It is an object of the present invention to provide a method for forming a metal oxide film using a sputtering process, where uniformity in characteristics of the metal oxide film to be formed on a wafer can be improved.
In a first aspect of the present invention, a method for forming a metal oxide film on a wafer on a stage facing to a target by a sputtering process, comprises the steps of: forming a metal film on a part of area on a surface of the stage, a part of area on a surface of the wafer, or both the part of area of the surface of the stage and the part of area of the surface of the wafer; connecting the metal film to a ground; and forming a metal oxide film on the wafer using the sputtering process while the metal film is being grounded.
In this aspect of the present invention, a metal film is formed on a part of area on the surface of the wafer, and the metal film is then connected to a ground, so that the surface of the wafer can be prevented from becoming charged at the time of performing the sputtering process. Or, a metal film is formed on a part of area on the surface of the stage, and the metal film is then connected to a ground. Thus, the potential can be consistent across the entire surface of the wafer and the ions can be uniformly flied on the metal oxide film at the time of performing the sputtering process. As a result, the metal oxide film can be formed to ensure uniform film characteristics.
The surface of the wafer may comprise a pattern area on which devices are arranged in a matrix and an outer area that surrounds the pattern area, and the metal film may be formed on at least the outer area in the step of forming the metal film. Alternatively, the metal film may be formed in a lattice pattern formed between devices on the pattern area in the step of forming the metal film. The metal film in the lattice pattern may be formed on cutting lines of the devices being arranged in the matrix.
The surface of the stage may comprise a wafer-mounting area to be covered with the wafer and a redundant electrode area external to the wafer-mounting area. In this case, the metal film may be formed on the redundant electrode area in the step of forming the metal film.
In a second aspect of the present invention, a method for forming a metal oxide film on a wafer on a stage facing to a target by a sputtering process, comprises the steps of: exposing a metal material constituting the stage by removing at least a part of a deposition film formed on the surface of the stage; connecting the metal material to a ground; and forming a metal oxide film on the wafer using the sputtering process while the metal material is being grounded.
Therefore, the influence of self bias potential on the redundant electrode area of the stage for the ions can be prevented. Thus, the ions can be uniformly flied on the metal oxide film at the time of performing the sputtering process. And, the metal oxide film can be formed to ensure uniform film characteristics.
The surface of the stage may comprise a wafer-mounting area to be covered with the wafer and a redundant electrode area external to the wafer-mounting area, and the deposition film formed on the redundant electrode area may be removed in the step of exposing the metal material being formed as the stage by removing at least the part of the deposition film formed on the surface of the stage. The removal of the deposition film may be performed using an etching process.
In a third aspect of the present invention, a method for forming a metal oxide film on a wafer on a stage facing to a target by a sputtering process, comprises the steps of: electrically suspending the stage by forming an area on which at least a part of an insulating film on the surface of the stage is exposed; and forming a metal oxide film on the wafer using the sputtering process under the conditions in which a potential of the surface of the wafer and a potential of the surface of the stage are substantially equal to each other. Therefore, the ions can be uniformly flied on the metal oxide film at the time of performing the sputtering process. And, the metal oxide film can be formed to ensure uniform film characteristics.
Also, the surface of the stage on which the wafer is placed may be substantially shaped like the surface of the wafer on which the metal oxide film is formed. Thus, the entire surface of the stage can be substantially covered with the wafer. In other words, on the surface of the stage, it is possible to substantially eliminate any area which is not covered with the wafer. As a result, the influence of self bias potential on the redundant electrode area can be prevented, ensuring the uniform potential distribution on the surface of the wafer. Therefore, the ions can be uniformly flied on the metal oxide film at the time of performing the sputtering process. In addition, the metal oxide film may be made of a bolometer material. The metal oxide forming the metal oxide film may have a resistivity of 1xc3x9710xe2x88x925 to 1xc3x97100 xcexa9xc2x7m. Furthermore, the metal oxide forming the metal oxide film may contain vanadium oxide.